The organic light emitting diode (OLED) display is one of the hotspots in the research field of flat panel display nowadays. Compared with the liquid crystal display (LCD), the OLED display has the advantages of fast response, high brightness, high contrast, low power consumption and easy to achieve flexible display etc., and is regarded as the mainstream display of the next generation. The pixel circuit is the core technical content of the OLED display, which has important research significance. Different from the LCD that uses a stable voltage to control the brightness, the OLED display is of current driven type, which requires a stable current to control the brightness. However, due to factors such as manufacture process and aging of the light emitting device, there may be nonuniformity in the threshold voltages Vth of the driving transistors in the pixel circuit, which may result in variation to the current flowing through each OLED such that the displaying brightness is nonuniform, thereby influencing the display effect of the whole image.